1. Field of the Invention
The present invention relates to voltage-controlled sine wave oscillators, and more particularly to a voltage-controlled capacitor used in such oscillators.
2. Discussion of the Related Art
FIG. 1 is a schematic of a conventional voltage-controlled sine wave oscillator 10 including a fixed capacitor Co coupled in parallel with a capacitor Cv whose value is fixed by a voltage Vc. The frequency of the oscillator is determined by the sum of the values of capacitors Co and Cv. Voltage Vc acts on both the value of capacitor Cv and the oscillator's frequency.
Generally, the capacitor Cv is a so-called "varicap" diode, the junction capacitance of which depends upon its bias voltage. However, such a variable capacitor has the drawback of being highly temperature-dependent.
PCT patent application WO 86/07508 describes a voltage-controlled capacitor formed with a circuit comprising two reference capacitors associated with two cross-coupled differential stages. The output voltage of each differential stage is applied to one terminal of the capacitor associated with the stage and the other terminal of the capacitor is connected to the emitters of transistors of the stage. To increase the limit operating frequency, compensation resistors are connected to cooperate with the reference capacitors. However, it is difficult to match a capacitor and a resistor in an integrated circuit, especially if the manufacturing technology of the integrated circuit is changed. Moreover, a capacitor and a resistor do not have the same temperature variation characteristics. As a result, the limit frequency of the controlled capacitor varies substantially as a function of the technology and of temperature.